Paper coating compositions are applied onto paper substrates in order to improve optical properties such as brightness, smoothness, gloss and ink receptivity of the finished coated paper and paperboard.
Coating compositions are generally applied in the form of water suspensions containing a pigment, i.e., clay, calcium carbonate, talc, titanium dioxide, zinc oxide, etc., a latex binder, a coating insolubilizer such as melamine-formaldehyde, glyoxal-urea resins, a lubricant such as calcium stearate, a pigment dispersant such as polyacrylate, a defoamer and a water retention aid acting as a thickener or rheology modifier.
The water retention aid controls water loss of the coating composition by slowing the migration of water into the paper substrate caused by capillary pressure, mechanical pressure and vacuum pressure during the coating process. Water retention aids also significantly change the rheological properties of the coating composition which can be used to control rheology-related coating properties such as coating weight, coating structure relating to pigment and latex flocculation and machine runnability.
Commercial water retention aids fall into three category types: alginates, cellulosics and polyacrylates. Alginates are linear anionic polymers containing gluronic and mannuronic units. They are typically used in paper coating compositions to hold pigments and binders on the surface of the paper substrate. The coating composition is applied onto the paper substrate at relatively low speeds and high solids content.
Cellulosics include carboxymethyl, hydroxyethyl and hydrophobically modified cellulose. The performance of cellulosics are formula dependent, but in general they exhibit good water retention properties and function as co-binders in paper coating compositions. Cellulosics come in a variety of grades (molecular weight, degree of carboxylation, etc.) and are more tolerant to calcium ions than alginates. They tend to form stable solutions over a broad pH range. Hydroxyethyl cellulose is reported to adsorb onto clays more so than carboxymethyl cellulose and, consequently, higher doses are required in order to obtain equivalent water retention properties.
Polyacrylates typically consist of copolymers of ethyl acylate and methacrylic acid and are considered alkali-swellable emulsions. In general, they can be made by copolymerizing acrylic (methacrylic) acid with a variety of other monomers. Polyacrylates are pH sensitive and must be used at a pH above 8 in order to thicken a paper coating composition. These alkali swellable emulsions are effective thickeners due to their relatively high molecular weight and extended chain configuration. However, due to their sensitivity to pH, salt content, and the presence of multi-valent ions in the paper coating composition, they can be difficult to control.